1. Field of the Invention
The present invention relates to a motor control apparatus which computes a display output to be fed to a load meter for viewing, the load meter being a device that shows the operating condition (load condition) of a spindle motor in a machine tool, wherein the motor control apparatus controls the motor by using the display output.
2. Description of the Related Art
To monitor the operating condition of a spindle motor in a machine tool or the like, motor load information is computed or detected. Conventionally, such motor load information is computed using factors that affect motor output, such as the output power of a motor control apparatus (also called an amplifier), the efficiency of the motor, and a torque command (or electric-current command) for the motor.
A spindle motor used to drive the spindle of a machine tool may be operated even in the nonlinear control region of the motor to achieve high-speed and high-output performance required of it. In that case, torque may not be produced as directed by the torque command, and as a result, the load meter value computed based on the torque command may deviate from the correct value. On the other hand, to monitor tool wear, etc., or to determine the cutting conditions of a machine tool, user demand for accurate display of load meter values is increasing.
Patent Document 1 discloses a method for visually displaying a load meter that enables the operating condition of a motor to be grasped instantly in real time. Generally, in prior art load meters, the load meter value is computed and displayed in the following manner to present the operating condition of a motor for viewing.
FIG. 1 is a block diagram showing the configuration of a first specific example of a motor control apparatus according to the prior art. In FIGS. 1 to 4 given hereinafter, the same reference numerals indicate the same parts. In FIG. 1, motor control apparatus 10 includes all the parts shown, except motor 1. As shown in FIG. 1, a speed control unit 12 for motor 1 receives an output of an adder 13 which shows the difference between the speed command and the actual speed V of motor 1, and outputs a torque command TC.
As shown in FIG. 1, a load meter 11 in prior art motor control apparatus 10 includes a load meter computing unit 111, a load meter display unit 112, and a storage unit 113. Efficiency η defined by a preset given value is stored in storage unit 113. Load meter 11 monitors the output of a driving unit 15, computes the output power of motor 1 from the output power W of driving unit 15 and the efficiency η of motor 1, and displays it as the load meter value on the display part of load meter display unit 112, or computes the ratio of the thus computed output power of motor 1 to the maximum output power at the corresponding rpm and displays it as the load meter value on the display part of load meter display unit 112.
LM, W, V, and η in the equation LM=f1(W, V, η) shown inside load meter computing unit 111 in FIG. 1 are defined as follows.                LM: Load meter value, W: Output power of amplifier, V: Actual speed of motor, η: Motor efficiency        
FIG. 2 is a block diagram showing the configuration of a second specific example of a motor control apparatus according to the prior art. A load meter 21 shown in FIG. 1 includes a load meter computing unit 211 and a load meter display unit 212. Load meter 21 monitors the torque command TC that a speed control unit 12 outputs (or the electric-current command that the driving unit outputs to control the current supplied to the motor), and displays (as the load meter value) the ratio (in %) of the torque command (electric-current command) TC to the maximum torque command (electric-current command) corresponding to the motor rpm (motor speed V) at that time.
LM, V, and TC in the equation LM=f2(V, TC) shown inside load meter computing unit 211 in FIG. 2 are defined as follows.                LM: Load meter, V: Motor speed, TC: Torque command        
[Patent Document 1] Japanese Unexamined Patent Publication No. H10-90011 (refer to [CLAIM 1] in the claims, paragraph numbers [0002] to [0004] in the specification, and FIG. 2)
However, in the case of the load meter shown in FIG. 2, when factors affecting the motor output, such as the torque command (electric-current command) used for controlling the motor, are used to calculate the load of the motor, if the motor enters a nonlinear region (for example, because of the saturation of the motor energization voltage or the saturation of the output voltage of the amplifier) the proportional relationship between the factors, such as the torque command, and the motor output will be lost. The resulting problem is that the load information (load meter) value calculated from these factors deviates widely from the desired value, causing a significant error.
On the other hand, the load meter computation method shown in FIG. 1 involves a problem that the load meter cannot be accurately calculated unless the motor speed and motor efficiency that varies with the load condition are accurately known, because the load meter computing value is affected by the amplifier output power and the motor efficient.